Police believe they know the identities of the men who planned the attack on Constable Ronan Kerr and made the bomb that killed him.

Constable Kerr, 25, died in a booby-trap car bomb in Omagh on 2 April 2011.

A senior detective involved in the investigation has told the BBC "significant progress" has been made in the case against the bomb-maker.

The police have also established links between those responsible and 16 other incidents.

These include the attempted murders of two other police officers.

Detectives believe the incidents were carried out by a group of about 20 to 25 members of the dissident republican group referred to as the new IRA.

It is said to consist of individuals from Omagh, Coalisland and the Toomebridge and Ballyronan areas along the shore of Lough Neagh, and in counties Monaghan and Louth in the Republic of Ireland, with their actions directed from Belfast.

36-year-old Gavin Coyle was sentenced to 10 years imprisonment for possession of explosives and firearms with intent to endanger life and membership of a proscribed organisation - the IRA. He will serve half of the term in custody and the other half on licence.

Coyle was arrested as part of the investigation into the murder of Constable Kerr.

Three days after the killing, police raided industrial units near Coalisland and found the largest haul of weapons and explosives in Northern Ireland for many years.

They included four assault rifles, ammunition, Semtex explosives, timer power units used to detonate bombs and a booster pack for an RPG 7 rocket launcher.

Rifle 'ready to use'

The police have said the search saved lives as there was evidence that further attacks were being planned.

The assault rifles were in what police describe as a state of readiness.

One had had been separated from the others, suggesting it was being prepared for use, and magazines containing the ammunition had been taped together to facilitate quick reloading.

Two cars stolen weeks earlier and found inside the unit had bottles of petrol in the back seats, suggesting they were to be used in an attack and then burnt out afterwards.

The police said the evidence suggested attacks using the rifles found in the unit were "highly likely".

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Media captionGavin Coyle will serve half his sentence in jail and the other half on licence

While Gavin Coyle was not charged with offences connected to the murder of Constable Kerr, the search was part of the huge investigation into the killing.

The police have said the links to other incidents mean it is the largest investigation in the history of the PSNI, with more actions than the Omagh bomb inquiry.

The scale is vast.

'Experienced bomb-maker'

At its height more than 100 detectives were involved. Nearly 8,000 items have been seized for potential forensic examination.

There have been 123 searches, and 14 arrests. More than 1,500 homes have been visited as part of inquiries.

The police seized and analysed CCTV footage from Omagh to Coalisland.

Nearly three years on, police believe they have now identified the man who made the bomb that killed the officer.

He is described as an experienced bomb-maker who lives in the Republic of Ireland.

Detectives also believe they have identified the man who led the bomb team, described as the driving force behind the plan.

A senior detective involved in the investigation has told the BBC that significant progress has been made in the case against the bomb-maker.

He said progress has also been made in cases against three other individuals for a series of serious offences linked to Ronan Kerr's murder.

Magnets stolen from taxi

The police say they have also made "significant progress" in terms of who acquired the component parts used to make the device.

They have established that the magnets used to attach the bomb under Ronan Kerr's car came from a sign stolen from a taxi in Omagh three weeks before the attack.

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Police said one assault rifle had been separated from the others and had ammunition magazines taped together for quick reloading

They have also established links between the dissident republican group they believe was responsible and 16 other incidents, including the attempted murders of two other police officers.

One of them was constable Peadar Heffron, a fluent Irish speaker and captain of the police GAA team, who was seriously injured when a bomb exploded under his car four years ago.

The group has also been linked to an attempted car bomb attack on the headquarters of the Northern Ireland Policing Board in November 2009.

Only the detonator exploded on a device containing 400 pounds of explosives.

A senior detective said Coyle's conviction and sentencing is far from the end of the investigation into Ronan Kerr's murder, but simply the end of a chapter in what will be a long story the PSNI hopes will end with further convictions.